The present invention relates generally to a branching unit for underwater cables that uses a double-coil relay. More particularly, the present invention relates to a branching unit for underwater telecommunication and power cables having a relay armature that is magnetically switched by a first coil and that is reinforced by a second redundant coil.
Underwater or submarine cable systems were originally designed to provide a telecommunications link between two landing points separated by a body of water, such as between France and England. Optical fiber within the submarine cable could carry high bandwidth telecommunications across tens of kilometers without the need for amplification or regeneration.
As applications evolved that required transmission across larger distances, optical repeaters in the form of regenerators or amplifiers were required within the cable span, which mandated the availability of electrical power. Power feed lines and optical transmission lines were provided together in the submarine cables. These cable systems were soon improved to provide a telecommunications connection to three or more separate landing points by employing a branching unit at the junction of multiple cables under the body of water. A standard branching unit connected a main cable from one landing point and two spur cables from second and third landing points, respectively, in a Y-shaped arrangement.
FIG. 1 illustrates a typical three landing point connection scheme. Branching unit 100 physically interconnects the cables, coordinates the routing of the cables, and provides for power switching between the cables, among other things. Conventionally, stations at the landing points power repeaters 120 within the three cables in a two-sided and one-sided arrangement. In particular, the transmission of electrical direct current along a power feed line of the intervening cables is established between any two of the landing points, and the third landing point passes electrical current through its respective cable to a ground at sea earth. Relays within the branching unit establish the two-sided (bilateral) and one-sided (unilateral) feeding configuration and help to change it if a fault arises in one of the branches. More elaborate schemes also exist for connecting multiple landing points and multiple branching units in a submarine network.
The act of configuring a branching unit refers to coordinating the application of power from the respective landing points by providing a first power connection (two-sided) between any two landing points and a second power connection (one-sided) between the third landing point and sea earth. The two-sided power connection between the above two landing points can be either a two-end feeding as shown in FIGS. 2A, 2B, i.e., a generator is present at each of the two landing points or, alternatively, a one-end feeding, i.e., a generator is present at one of the landing points and the cable is grounded at the other of the two landing points.
FIGS. 2A and 2B illustrate the two configuration steps. As shown in FIG. 2A, a low current is caused to flow from landing point A to landing point C via branching unit 100. In this arrangement, the line from A to C is nominated as the main trunk, and the spur branch to landing point B is left in an open-circuit condition. FIG. 2B shows that after the main trunk has been established, the one-sided powering of the spur takes place by shunting the spur to sea earth at the branching unit and feeding the spur from landing point B.
Various publications describe this configuration process. U.S. Pat. No. 5,196,984 discloses a branching unit which employs electrical power feeding for repeaters and multiple branching units. The branching unit terminates three line cables and a sea earth and includes three high voltage element relays, only one of which is energized at a time. When electrical power is supplied between any two line cables, the third is isolated and connected to the sea earth. Short circuit or open circuit faults in one line cable may be isolated and connected to the sea earth, while allowing powering of the remaining two line cables that are free of faults.
Similarly, U.S. Pat. No. 5,214,312 discloses a power feed line switching circuit for a submarine branching unit having first, second and third electrical paths connected in a Y-shaped connection, and first, second and third terminals connected respectively thereto. The power feed line switching circuit also has first, second and third relays each including a drive unit inserted in the first, second and third terminals and a switching unit for disconnecting the corresponding terminal and connecting the terminal to the ground. The relays control the connections between the terminals and electrical paths for establishing a one-end power feed line or a two-end power feed line to maintain power feed for repeaters and the submarine branching unit by the plurality of relays.
U.S. Pat. No. 5,644,466 discloses a submarine cable branching system including a plurality of directional relays in respective power feed paths branched at a common node. Each of the directional relays is energized by a current flowing through the power feed path in a predetermined direction for energizing a corresponding switch provided in a different power feed path to establish a bilateral feed path and further a unilateral feed path. A bypassing switch is provided across one of the switches that forms the bilateral feed path such that the bypassing switch is urged to close in response to energization of a self-sustaining relay provided in the unilateral feed path.
Since the branching unit, repeaters and cables are laid underwater and are thus difficult to maintain, it is important that they have a high reliability to ensure uninterrupted telecommunications between the landing points. During power-up of the main trunk, however, the spur cable will acquire a charge related to the voltage drop at the branching unit and the length of the spur. Switching the spur cable to sea earth (typically the sea water) can cause a rapid discharge, which may damage contacts and relays within the branching unit. In particular, a relay that causes the spur cable to contact the sea earth is typically a high-voltage changeover relay. When actuation of the relay causes a make of the switch, an arc discharge will liberate a quantity of contact material. Acting as charge carriers, the liberated material may pass between the just opened contacts in the three-terminal relay. An avalanche effect could take place that causes the main cable to discharge across the contact gap with large amperage, leading to imminent failure or degradation of the relay and connection of the main cable to sea earth. In this scenario, either it could be impossible to power the system up and configure the system or an oscillation could be initiated between the main trunk and spur (see, e.g., U.S. Pat. No. 5,517,383 at columns 1 and 6).
U.S. Pat. Nos. 5,517,383 and 5,841,205 propose circuits to avoid this damage. In the ""383 patent, an additional relay D and switch Dl are used to avoid dangerous arcing in the high voltage changeover relay such as C. Relay D is not a changeover relay and can withstand a greater amount of arcing without failing.
The ""205 patent discloses the use of a two-stage relay to avoid damage from arcing. In this arrangement, the spur is disconnected from the main trunk when a defined current level is reached and then discharged by shorting its terminal station end (i.e., not underwater). After current in the main trunk ramps to a second level, the spur is connected to sea earth at the branching unit without arcing.
U.S. Pat. No. 4,798,969 discloses a branching unit circuit that includes an auxiliary relay that prevents the spur branch from re-connecting to the main trunk when its current falls to zero. A relay K1 in the main trunk controls the switch connecting the spur branch to sea earth. A latch-up relay K4 in the spur branch holds the spur branch out of contact with the main trunk if relay K1 opens and relay K4 remains energized. This configuration, however, requires additional relays and switches in the branching unit.
Applicants have found that these prior arrangements introduce unnecessary complexity and expense to a branching unit and its operation.
Applicants have observed that the above problem can be overcome with a branching unit that includes a relay with two separate coils, rather than a single coil. The relay can be energized by the magnetic field generated by either coil, and this magnetic field can be reinforced when currents of proper direction flow in both coils. Thus, once the relay is configured by the current flowing in one coil, the magnetic field that sets the configuration may be reinforced by sending current of proper direction through the other coil. Therefore, even if the current in the first coil should stop flowing, the relay will remain configured by the magnetic field generated by the second coil. Applicants have also observed that a branching unit that uses such a double-coil relay exhibits improved reliability and reduced relay failure.
In one aspect, a branching unit for use in submarine telecommunications systems consistent with the present invention includes first, second, and third cable terminations each coupled to a power feed line of a respective submarine cable, a ground termination, and first, second, and third high-voltage relays. The high-voltage relays each have a first coil and a first contact. The first coil of each high-voltage relay is positioned between two of the cable terminations respectively and has an energized state when a first threshold amount of current passes through the first coil and a de-energized state when a second threshold amount of current does not pass through the first coil. The first threshold amount of current for causing the energized state is greater than the second threshold amount of current for causing the de-energized state. The first contact of each high-voltage relay is positioned to connect the respective third cable termination with the ground termination when the respective first coil is in an energized state. At least one of the high-voltage relays includes a second coil in series with the respective third cable termination and the ground termination. The first contact is positioned to maintain the connection between the third cable termination and the ground termination when current exceeding a third threshold passes through the second coil. The first contact also connects the respective third cable termination with the other two cable terminations when both the first coil and the second coil are in a de-energized state.
Preferably, the first, second, and third high-voltage relays further include a second contact positioned to isolate the coils of the other two relays from each other when the respective first coil is in an energized state. More preferably, the first, second, and third high-voltage relays are arranged between the cable terminations in a delta network.
In another aspect, a branching unit for joining power feed lines of at least three submarine cables consistent with the present invention has first, second, and third cable terminations each coupled to a power feed line of a respective submarine cable, a ground termination, and a high-voltage relay. The high-voltage relay has a first coil in series with two of the cable terminations and a second coil in series with the third cable termination and the ground termination. The high-voltage relay includes at least one contact positioned to connect the third cable termination to the ground termination when electrical current in the first coil exceeds a first predetermined threshold. Further, the contact is positioned to disconnect the third cable termination from the ground termination and reconnect it to the other two cable terminations when electrical current in the first coil falls below a second predetermined threshold and electrical current in the second coil falls below a third predetermined threshold.
Preferably, the first predetermined threshold is about 60-100 mA for the first coil. Also, preferably the respective second and third predetermined thresholds for the first and second coils are 10-50 mA and 300-350 mA.
In still another aspect, an underwater optical telecommunication link according to the present invention comprises at least three submarine cables extended at least partly in a body of water and having first, second and third power feed terminations at respective landing points, each cable comprising at least an optical fiber and a power feed line electrically connected to the respective power feed termination; at least two power stations at the landing points to feed said power feed lines of the submarine cables; and a branching unit according to one of the other aspects of the invention, in said body of water, for joining said power feed lines of said submarine cable.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the invention as claimed. The following description, as well as the practice of the invention, set forth and suggest additional advantages and purposes of the invention.